Photoreceptors conventionally widespread in electrophotography are those comprising inorganic photoconductive materials, such as selenium, selenium-tellurium alloys, selenium-arsenic alloys, and cadmium sulfide.
Organic photoreceptors comprising organic photoconductive materials have also been studied with attention being paid on their merits over inorganic photoreceptors, such as inexpensiveness, productivity, and ease of disposal. In particular, organic photoreceptors of separate function type having a laminate structure composed of a charge generating layer which functions to generate charge on exposure to light and a charge transporting layer which functions to transport the generated charge are excellent in electrophotographic characteristics, such as sensitivity, charging properties, and stability of these properties on repeated use. Various proposals on this type of photoreceptors have been made to date, and some of them have been put to practical use.
While organic laminate type photoreceptors exhibiting satisfactory performance in terms of the above-mentioned electrophotographic characteristics have been developed, there still remains an unsolved problem of durability against mechanical outer force in nature of the organic material used. That is, being made of an organic material, an organic photosensitive layer easily undergoes wear or scratches on direct imposition of loads from a toner, a developer, a transfer medium (e.g., paper), a cleaning member, and the like and tends to suffer from adhesion of foreign substances due to, for example, a toner filming phenomenon, which results in image defects. Besides, low-resistant substances, such as corona discharge-induced ozone and nitrogen oxides, and paper dust from copying paper are liable to be deposited on the surface of the photoreceptor, which leads to image diffusion under a high temperature condition. The working life of an organic photoreceptor has been considerably limited by these phenomena.
On the other hand, it has been demanded to achieve speeding up in formation of a color image with full color copying machines and color printers.
A number of measures have ever been proposed to improve durability of an electrophotographic photoreceptor. For example, various polycarbonate resins have been suggested as a binder resin for the surface layer of a photoreceptor (see JP-A-60-172044, JP-A-62-247374, and JP-A-63-148263; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Various charge transporting materials to be used in the charge transporting layer have also been proposed. For example, JP-B-59-9049 (the term "JP-B" as used herein means an "examined published Japanese patent application") and JP-A-62-247374 disclose benzidine compounds, and JP-A-2-178668, JP-A-2-190862, JP-A-3-101739, and JP-A-3-127765 disclose triarylamine compounds.
While durability of a photoreceptor can be improved by a proper choice of the binder resins proposed, the improvement so far achieved is still unsatisfactory. That is, even if the coating film made of a known binder resin has sufficient mechanical strength, once toner filming occurs on a photoreceptor as a result of insufficient cleaning, image quality cannot be restored unless the photoreceptor is exchanged for a new one. Further, some binder resins proposed for improvement of durability impair photosensitivity of a photoreceptor.
The charge transporting materials proposed to date, when used repeatedly, suffer from reduction in charge potential and increase in potential at an exposed portion and also cause fog, resulting in deterioration of image quality.
The demand for high image quality has been increasing as is seen from the recent advancement of full color copying machines, and it has been getting difficult for known techniques to meet the demand. Therefore, it has been demanded to improve cleanability of a photoreceptor. Further, since many steps and expenses would be required for eliminating minute defects occurring in the photosensitive layer of a photoreceptor, the manufactures have been compelled to accept occurrence of defects to some extent so as to supply their products at competitive prices. It follows that copies obtained suffer from fine black spots on the white background or fine blank areas on the image area. These image defects may no more be ignored in the case where an image extends all over paper like full color copies.